One may not be skilled in the habit of other's throughts but at least one can make this resolve: "I will be skilled in the habit of my own thoughts." This is how you should train yourself, and this is how it is done. A woman, a man or a youth fond of self-adornment, examining his reflection in a bright, clear mirror or a bowl of clear water might see a blemish or a pimple and try to remove it. And when he no longer sees it there, he is pleased and satisfied and thinks: "It is an advantage to be clean."
In the same way, one's introspection is most fruitful in good states when one thinks: "Am I usually greedy or hateful, overcome by sloth and torpor, with excited mind or filled with doubt or anger, or am I not? Do I usually live with soiled thoughts, or clean thoughts? With body passionate or not, sluggish or full of energy, uncontrolled or well controlled?"
If on self-examination one finds that he does live with these evil unprofitable states, then he must put forth extra desire, effort, endeavor, exertion, energy, awarenees and attention to abandon them.
And if on self-examination he finds that he does not live with the evil and unprofitable states, he should make an effort to establish those profitable states and further destroy the defilements.
[...from the April 20th entry in the Digital Buddha Vacana, a digital version of the book Buddha Vacana: Sacred Literature of Buddhism. I use the Palm OS version but it is available online and you can download it for other platforms.]